1. Field of the Invention
The present invention relates to a television imaging system for a guided missile or the like which provides improved target contrast and guidance reliability.
2. Description of the Related Art
Air-to-ground missiles (AGMs) such as the Maverick missile utilize a television system for guiding the missile to a designated target. A television camera is mounted in the front of the missile and generates a composite video image of the field of view in front of the missile. This image is initially displayed on a cathode ray tube (CRT) in the aircraft cockpit. The pilot or weapons system operator positions crosshairs on the CRT over the designated target, initiates lock-on, and then presses a button to launch the missile. A guidance electronics system in the missile converts the composite video image from the television camera into a digital image, which is analyzed using a centroid tracking or other algorithm to automatically guide the missile to the target. Deviations of the missile flight path from the designated path to the target are detected, and error signals are generated which are applied to deflect movable flippers or fins which guide the missile back toward the designated path.
Television cameras used in AGMs have previously utilized a Vidicon.RTM. camera tube which was initially manufactured in the late 1940s by RCA. The AGM variant Vidicon.RTM. was originally manufactured by General Electric, and is presently manufactured by MPD, Inc. of Owensboro, Ky. The Vidicon.RTM. tube has a photosensitive target layer onto which an optical image of the missile field of view is focussed by a lens assembly. The photosensitive layer is formed of antimony trisulfide (Sb.sub.2 S.sub.3), and has a spectral sensitivity or wavelength range which is close to that of the human eye, extending from approximately 400 to 800 nanometers.
The Vidicon.RTM. tube is described in a textbook entitled "TELEVISION ENGINEERING HANDBOOK", edited by K. Benson, McGraw Hill, 1985, section 11.4 entitled "Vidicon Type Camera Tubes", pp. 11.26-11.28. The Vidicon.RTM. tube has characteristics which are far from ideal for guided missile applications, including low sensitivity, low gamma, restricted operating temperature range, high dark current, high image lag, and poor spectral response. The low sensitivity, low gamma and spectral response limitations of the Vidicon.RTM. tube provide relatively low contrast between the target and background surroundings, resulting in low target tracking and missile guidance reliability. These drawbacks render the missile system unusable during approximately 40% of daylight hours in Northern European winters.